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To make it a user capable of performing sudo, add him to the sudo group using either of the following commands:

sudo usermod -a -G sudo <username>
sudo adduser <username> sudo

This works because the sudo group is predefined in /etc/sudoers. Note though that older versions of Ubuntu will use admin as group instead:

Until Ubuntu 11.10, the Unix group for administrators with root privileges through sudo had been admin. Starting with Ubuntu 12.04 LTS, it is now sudo, for compatibility with Debian and sudo itself. However, for backwards compatibility, admin group members are still recognized as administrators